Introduction
It has long been considered by oceanographs that nitrogen is the major
limiting nutrient of primary productivity throughout much of the
ocean whereas phosphorus is the limiting element for freshwater plankton productivity. However, recent observations show that other elements than nitrogen could limit primaty production in seawater.
Among the most often cited, phosphorus is suggcsted in coastal waters of
China (1 larrison et al, 1990), of Chcsapeakc Bay (Fisher et al, 1992), of
Sandsfjord (Thingstad et al., 1993) and in the Mediterranean oligotrophic
waters (Krom et al., 1991), silicium in coastal waters (Del Amo, 1996) and
iron in the ecjuatorial Pacific ocean (Martin, 1992; Coale et al, 1996) and
the Southern (Ocean (De Baar et aL, 1995).
Studies carried out in May 1995 within the Gironde plume waters in
the bav of Biscay showed that spring phytoplankton blooms displayed
atypical characteristics for blooms in temperate waters. They were already severely phosphorus-limited, considering the very' high values of
NO, N/P0 4 P ratios and the high C/P and N/P ratios for thc particulate organic matter (Herbland et al., 1998). They were dominated by
phytoplankton cells below 3 pm and were submitted to a strong grazing by microzooplankton (Sautour et al., 2000).
The objective of the present study is to test validity of this previous
hypothesis, “classical” phytoplankton blooms dominated by large cells
should occur earlier in the year. They should be responsible for the
early exhaustion of phosphate with important consequences for the
structuration of phytoplankton in spring towards the dominance of
small cells.
Material and methods
Sampling strategy
Cruises were carried out aboard R/v ‘Thalassa” in 1998: Biomet II (821 Januarv), Biomet III (25 February - 11 March) and Pqtgse (
7 -21 |unc)
and aboard R/V’ "Thalia" in 1999: Plagia 1 (21-28 February), Plagia 2
(24-30 April) and Plagia 3 (25 May-1 June). Stations were chosen according to the spread and characteristics of the plume from the continuous
subsurface temperature and salinity record by an on board thermosalinograph, the predictions of the 3D hydrodynamical model (Lazure &
Jégou, 1998) and SeaWIFS imagety (Froidefond etal, submitted).
We use the terms “Gironde” and “Oceanic” in the following sections
when we refer to stations within surface salinity range 32-34.5 and stations achieved at the limit of the Gironde plume (surface salinity
range 34.5 - 35.6), respectively.
Methods
For dissolved nutrients determination, seawater samples were filtered
on glass fiber filters (Whatman GI
; /F) with a syringe filtration system.
120
It has long been considered by oceanographs that nitrogen is the major
limiting nutrient of primary productivity throughout much of the
ocean whereas phosphorus is the limiting element for freshwater plankton productivity. However, recent observations show that other elements than nitrogen could limit primaty production in seawater.
Among the most often cited, phosphorus is suggcsted in coastal waters of
China (1 larrison et al, 1990), of Chcsapeakc Bay (Fisher et al, 1992), of
Sandsfjord (Thingstad et al., 1993) and in the Mediterranean oligotrophic
waters (Krom et al., 1991), silicium in coastal waters (Del Amo, 1996) and
iron in the ecjuatorial Pacific ocean (Martin, 1992; Coale et al, 1996) and
the Southern (Ocean (De Baar et aL, 1995).
Studies carried out in May 1995 within the Gironde plume waters in
the bav of Biscay showed that spring phytoplankton blooms displayed
atypical characteristics for blooms in temperate waters. They were already severely phosphorus-limited, considering the very' high values of
NO, N/P0 4 P ratios and the high C/P and N/P ratios for thc particulate organic matter (Herbland et al., 1998). They were dominated by
phytoplankton cells below 3 pm and were submitted to a strong grazing by microzooplankton (Sautour et al., 2000).
The objective of the present study is to test validity of this previous
hypothesis, “classical” phytoplankton blooms dominated by large cells
should occur earlier in the year. They should be responsible for the
early exhaustion of phosphate with important consequences for the
structuration of phytoplankton in spring towards the dominance of
small cells.
Material and methods
Sampling strategy
Cruises were carried out aboard R/v ‘Thalassa” in 1998: Biomet II (821 Januarv), Biomet III (25 February - 11 March) and Pqtgse (
7 -21 |unc)
and aboard R/V’ "Thalia" in 1999: Plagia 1 (21-28 February), Plagia 2
(24-30 April) and Plagia 3 (25 May-1 June). Stations were chosen according to the spread and characteristics of the plume from the continuous
subsurface temperature and salinity record by an on board thermosalinograph, the predictions of the 3D hydrodynamical model (Lazure &
Jégou, 1998) and SeaWIFS imagety (Froidefond etal, submitted).
We use the terms “Gironde” and “Oceanic” in the following sections
when we refer to stations within surface salinity range 32-34.5 and stations achieved at the limit of the Gironde plume (surface salinity
range 34.5 - 35.6), respectively.
Methods
For dissolved nutrients determination, seawater samples were filtered
on glass fiber filters (Whatman GI
; /F) with a syringe filtration system.
120